The present invention relates to a detector of phase modulated signals and, more particularly, to a phase demodulator particularly for use with a telemetry receiver for reception of signals from space and other weak and noisy signal sources.
In recent years, the use of radio links for transmission of data has expanded greatly in such fields as antenna tracking, communication satellites, meteorological satellites and related fields. The increased use of such data links in areas requiring the utmost in system reliability and performance demanded the development of new equipments and techniques to assure reliability of the data extracted from weak noisy signals; signals, for instance, in which the carrier signal is as much as 20 dB below the noise level.
It is essential to demodulation of phase modulated signals to be able to lock the receiver onto the carrier whereby to provide a stable phase reference signal against which the phase modulated signal is to be compared. With carrier levels received from deep space as low as 20 dB below noise, acquisition is difficult to achieve and to hold once achieved. Additionally, the time required to acquire lock is often excessive particularly where narrow bandwidths (30 Hz) are involved and thus the ability to hold lock, once acquired, is critical.